7th February 1995

From
Michael Ashley.....

Just when we thought everything
was working smoothly, the IRPS mirror rotator begins to act
strangely. When driving it from zenith to nadir it vibrates
alarmingly and skips steps. Thinking the problem could be
a missing phase, John spends some hours checking for broken
cables and electrical problems (it is not easy removing the
backshell from a connector on the roof). This time is considerably
extended due to John's need to persuade the LeCroy 300 MHz
Digital Sampling Oscilloscope that all he requires is a simple
trace of voltage versus time rather than a fourier transform
or cross-correlation with something it has previously measured.
The LeCroy has a brain the size of a small planet, and is
reluctant to do something as basic as displaying a simple
waveform, without first providing the operator with every
opportunity to sample its prodigious capabilities. Only after
John's threat to unplug it does the LeCroy agree to masquerade
as a dual-beam oscilloscope.

All connections to the motor check
out OK, and the drive electronics is also fine. We then begin
to suspect a resonance between the rate at which we are driving
the motor, and its natural torsional period. By trying various
inter-pulse delays, we find that at practically any rate other
than the one we were using (30 milliseconds between pulses)
the motor behaves perfectly. It appears that a slight change
in the calibration of the software timing loop, and a small
change in the motor driving voltage (due to using a UPS),
conspired to bring the rotator into resonance. Resolving this
problem consumed almost all of the day, and left us somewhat
concerned about what new surprises IRPS had in store for us
with only 24 hours left before we had to leave the Pole.

Dinner was superb: lobster tails and
steak and no ``toxic purple'' in evidence. Coincidentally,
three senior NSF representatives were visiting for the day.

Jean is analysing the data from his
first two balloon launches, and is getting ``seeing'' figures
of around 1 arcsecond. Jean is also excited to discover that
he has won a lottery to travel back to Christchurch aboard
the vessel Greenwave. The trip will take 6 days.

John is pursuing his goal of videotaping
the remaining women on the base, and manages an extended session
with Emily, the cook. (John says it is important that our
video properly acknowledges the vital role played by women
in the running of the South Pole base. Next time, he can write
the software and I'll video the women.)

Erik completes his goal of being the
fastest person to unicycle around the globe, completing all
360 degrees of longitude in about 10 seconds.

While using the computer to fill
the inner can of IRPS, John notices that when the can is full
a great spray of LN2 comes gushing out of the pressure relief
valve. I decide to use this as an indication of fullness,
and install a silicon diode in a suitable place to be hit
by the LN2. An intermittent solder joint in the control box
thwarts by first attempts, but in the early hours of Wednesday
the system is working smoothly. We can now sense when the
inner can needs filling (by monitoring the detector temperature)
and when the can is overfull (using the newly installed diode).
The IRPS computer now keeps track of the time at which each
fill occurred, and for how long the LN2 solenoids were open.
With experience we should be able to use this information
to decide how full the Taylor-Wharton dewar is, and to alert
Jamie to the need to refill it.